V. PIGMENT SYNTHESIS INHIBITORS
These herbicides affect the synthesis of photosynthetic pigments. As a result, the affected parts of the
plant become white or translucent. There are only two families in this group of herbicides: pyridazinones
with norflurazon as the main representative of the family and isoxazolidinones in which clomazone is
most used at present.
Norflurazon inhibits the desaturation of phytoene [108,109], which is the first step in the synthesis
of carotenes from geranylgeranylpyrophosphate (GGPP) (see Figure 7) [110]. The primary action of this
family of herbicides is interference with carotenoid biosynthesis at the level of phytoene desaturase
[6,111,112]. The inhibition of this enzyme results in a decrease in carotenoids together with chlorophyll
and a concurrent accumulation of phytoene [110,111,113].
It is not known where clomazone acts, but it is certainly different fromt the site of action of norflu-
razon [114,115].
Herbicides of these two families are absorbed by roots and move via the xylem to the leaves. Devel-
oping leaves tend to be so sensitive to these herbicides that very small quantities are sufficient for their
bleaching. These herbicides do not have significant agronomic interest because they have not been ex-
tensively studied and their resistance mechanisms are still unknown. A norflurazon-resistant mutant has
been isolated from the alga Chlamydomonas reinhardtiiwith alterations in the target enzyme phytoene
desaturase [116]. Resistance to bleaching herbicides affecting phytoene desaturase has been obtained in
cyanobacteria by genetic engineering [117].
VI. GROWTH REGULATORS
A series of herbicide families affect the growth of crop plants. Among these families are found phenoxy-
acetic acids, benzoic acids, and pyridines.
Their action sites are not exactly known, but they can act at several sites in a plant. For example, they
disrupt hormone balance and protein synthesis and thereby produce a variety of growth abnormalities.
782 DE LA ROSAFigure 7 Scheme of -carotene synthesis from phytoene with desaturases and cyclase. Dashed arrows indi-
cate the target for herbicides pyridazinones and (probably) isoxazolidinones.